Here is the tree for all adorable electric rodents, including the world's most iconic Pokémon of all! Naturally I used Pikachu as the base for the shared ancestor because, well, it would be criminal not to!

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That's what Coolsteph said, but Pikachu is so iconic that I decided the ancestor should look a bit like him. If a big Pokémon evolving into two small Pokémon is really that large of a problem, the Emolga-Pachirisu ancestor and the Plusle-Minun ancestor can both have a small-bodied shared ancestor that evolved from the Pikachu ancestor.